Gasification of carbonaceous solids



March 16, 1954 w MATTOX 2,672,410

GASIFICATION OF CARBONACEOUS SOLIDS Filed D60. 1, 1949 EXCHANGER O2 INLTSTEAM INLE7" Patented Mar. 16, 1954 2,672,410 .GASIFIGATION OFCARBONACEOUS SOLIDS William J. Mattox, Baton Rouge, La.,assignor toStandard Oil Development Company, a corporation of Delaware ApplicationDecember 1, 1949, Serial No. 130,493

15 Claims.

The present invention relates to the production of gases fromnon-gaseous carbonaceous material, and specifically to the production ofgas mixtures containing carbon monoxide and hydrogen, such as water gas,from such carbonaceous materials as coke and coals, wherein theformation of carbon dioxide is repressed by the addition of an alkalimetal halide.

It has long been known that solid fuel materials, such as coke, coal,and the like, may be converted into more valuable gases which can moreeasily be handled and more efficiently utilized for a greater variety ofpurposes. One of the most widely practiced gas-generating conversions isthe so-called water-gas process in which solid fuels, such as coal orcoke of any origin, are reacted with steam at temperatures of about1400" to 2000 F. to produc water gas mixtures of carbon monoxide andhydrogen in varying proportions, depending mainly on the time ofcontact, conversion temperatures, and steam feed rate. The overall watergas reaction being endothermic, heat must be supplied; this is usuallyaccomplished by the combustion of a portion of the carbonaceous feedwith an oxidizing gas, such as oxygen, at about 1600-2400 F. Thecombustion reaction may be carried out either simultaneously with thewater gas reaction or alternately in a make-and-blow fashion.

The water gas process permits the production of gas mixtures of varyingcomposition and B. t. u. content. The process as such, therefore, issuited not only for the production of fuel gases but also for theproduction of gases for hydrogenation processes and particularly for thecatalytic synthesis of hydrocabons and oxygenated organic compounds fromC and H2, which process requires H2:CO ratios, depending on the productsdesired and reaction conditions to be maintained, varying within limitsof about 0.5 to 2.5 or more volumes of H2 per Volume of CO.

The technical utilization of the water gas process, particularly forhydrogenation and for production of synthesis gas, has been impeded bydifficulties encountered particularly in heat supply, continuity ofoperation, and limitations in temperature imposed by low ash fusion orsoftening points. The problem of continuity of operation has beensatisfactorily solved heretofore by the application of the fluid solidstechnique wherein the carbonaceous charge is reacted in the form of adense turbulent mass of finely divided solids fluidized by the gaseousreactants and products. Heat is generated either by partial combustionof carbonaceous materials within the ga generator in a so-calledsingle-vessel system or a continuous circulation of suspended solidcarbonaceous material to a separate heater in which heat is generated bycombustion of the carbonaceous constituents of the residue, andrecirculation of the highly heated fiuidizable combustion residue to thegas generation zone to supply the heat required therein in a so-calledtwovessel system.

The single-vessel system would as such; be more desirable than thetwo-vessel system because the latter requires the circulation oftremendous quantities of solids between the two vessels, a factor whichpresents serious problems of design and equipment maintenance. However,single-vessel operation involves the disadvantage of product gasdilution with nitrogen and carbon dioxide when air is used as thecombustion-supporting gas. Since technically pure oxygen has becomeavailable at relatively low cost, nitrogen dilution may be eliminated bythe use of oxygen in a commercially feasible operation. However, carbondioxide formation remains a problem seriously affecting thepracticability of the otherwise preferable single-vessel system. Recentinvestigations have shown, for example, that in a gasification operationemploying 2300 lbs. of powdered coal, 1700 cu. ft. of oxygen and 2000lbs. of steam per hour to produce 70,000 cu. ft. of water gas, theproduct gas contains about 16% of CO2. This carbon dioxide represents atotal loss as faras the output of hydrogen or synthesis gas isconcerned. The significance of this loss will be appreciated when it isborne in mind that the present methods of synthesis gas productionaccount for about of the cost of synthetic fuel production. Although CO2formation may be reduced to a certain extent by adjusting operatingconditions, this can be done only at the expense of generator capacity.

The present invention substantially alleviates these difficulties andafiords various additional advantages as will appear from thedescription given below wherein reference will be made to theaccompanying drawing.

In accordance with the present invention, carbon dioxide formation isconsiderably reduced and even substantially eliminated in water gasgenerators wherein carbonaceous solids are gasified with steam andoxygen, by the addition to the generator of small proportions of ahalide which at the reaction conditions yields halogen and/or a volatilehalogen compound.

, 'Alkali metal halides, such as the chlorides, bromides, and fluoridesof sodium, potassium, lithium, rubidium, and cesium, or mixtures of suchhalides are particularly suitable for the purposes of the invention, butalkaline earth metal halides, such as those of barium, calcium, andmagnesium, may also be used. Sodium chloride, because of its readyavailability and low cost, is the preferred addition agent. Proportionsof about 0.015%, preferably about 0.05-l%, by weight of alkali metalhalide based on carbon supplied to the generator, are suitable todepress C02 formation from about 15-20% to less than about 5% of theproduct gas.

While it is not intended to limit the present invention by any theory ofthe reaction mechanism involved, it is believed that the effect of theaddition of halides may be explained as follows. Alkali metal halides,such as NaCl, react with steam in the presence of oxygen at the gasgeneration temperatures of about 1800-2000 F., to yield free halogenand/or hydrogen halide and alkali metal oxide and carbonate. Smallamounts of halogen or volatile halogen compounds are known to suppressthe formation of CO2 in favor of an exclusive formation of CO in thecombustion of carbonaceous materials. Since CO2 formation in the watergas generator is largely due to a complete combustion of carbon to CO2,the

halogen or halogen halide supplied by the alkali metal halide will actto suppress CO2 formation in the water gas reaction.

It will be appreciated from the foregoing that the addition of halidesin accordance with the invention has nothing in common, regarding itspurpose and effect, with the known addition of such agents catalyzingthe water gas reaction as alkali metal carbonates, nickel, heavy metaloxides or sulfides, etc., or mixtures thereof which have no inhibitingeffect on the formation of 00:. It is, however, a particular advantageof the present invention that such catalytic agents as alkali metaloxides or carbonates are formed as the solid residue of the liberationof halogen or hydrogen halide. As a result of this secondary catalyticeiiect, the gasification temperature may be substantially reduced andthe 00 content of the product gas considerably increased. The lattereffect is due mainly to an acceleration of the reaction which may bevery appreciable at NazCOa concentrations as low as 0.01%, as has beenshown by other researchers.

In accordance with the preferred embodiment of the invention, the alkalimetal halide to be added is introduced into the gas generator by theinjection of, or the impregnation of the carbonaceous charge withnatural brines containin the desired halide, for example sea water. Thealkali metal oxide or carbonate remaining on the gasification residuemay be recovered as the corresponding hydroxide or carbonate by anaqueous extraction of the gasification residue. The aqueous extract maybe used as a scrubbing solution to remove any excess halogen from theproduct gases, if desirable. The halide salt content of the usedscrubbing solution may be reutilized in the process as a carrier ofalkali metal halide to be added to the generator in accordance with thebasic principle of the invention. In some cases, it may also bedesirable to pass a certain quantity of halogen with the gas produced toa synthesis stage in order to promote a selectivity more favorable tocertain desired products, in-

4 control the Hz/CO consumption ratio of the synthesis reaction.

The process of the invention may be carried out in any system whereincarbonaceous solids are reacted with steam and oxygen to produce gasmixtures of the type of water gas. Any conventional gas generatorworking on this principle may be adapted for the purposes of theinvention, independent of the special technique involved, i. e. theessential advantages of the invention may be realized in fixed bed,moving bed, fluid bed or true suspension operation. However, fluidoperation is preferred, because of its superior characteristics ofgas-solids and solids-solids contact and its improved heat transfercharacteristics, which greatly enhance the inhibiting and catalyticeffects of the addition agents of the invention. A system of thispreferred type is illustrated in the drawing, the single figure of whichdepicts schematically an expanded flow plan of the process.

Referring now to the drawing, the system shown consists essentially of aconventional fluid-type water gas generator [0 and metal haliderecovering equipment 30 and 40, the functions and cooperation of whichwill be forthwith described using the gasification of a low temperaturecoke in the presence of NaCl as an example. It should be understood,however, that the system may be operated in a similar manner for thegasification of other carbonaceous solids, such as various coals, in thepresence of other suitable halides.

In operation, fresh coke ground to a particle size passing 4 mesh with amajor proportion passing 60 mesh is fed from a feed hopper, preferably alock hopper I, into a standpipe or other conveying means 3 intogenerator Ill. The coke may be preheated to temperatures of about 400-800 F. in hopper I by means of product gases passed therethrough vialines 6 and 8, or in any other conventional means. Sea water isintroduced through line 5 into standpipe 3 in amounts sufiicient tointroduce about 0.054% by weight of NaCl, based on carbon, into thecoke. About 25-100 gals. of sea water per ton of coke passing throughstandpipe 3 are normally adequate for this purpose. If the coal ispreheated as described, the sea water evaporates quickly in thestandpipe and the steam so generated acts as an aerating agent.

Simultaneously, a mixture of steam and Oz in a ratio of about 700-1800lbs. of steam per 1000 cu. ft. of O2 is supplied from lines 12 and M,respectively, through a distributing device such as grid 16. About1000-1500 cu. ft. of oxygen and about 700-2400 lbs. of steam per ton ofcoke are normally sufiicient to support the water gas reaction in thepresence of NaCl at temperatures of about 1'7001800 F. In the absence ofNaCl, the steam and oxygen consumption are somewhat higher for theproduction of a gas of comparable composition, and the process is muchmore sensitive to variations in the concentration of these reactantsthan one employing the alkali halide additive. Reactor [0 should be sodesigned that at the solids and gas feed rates involved a linearsuperficial gas velocity of about 0.5-1.5 ft. per second is maintainedin reactor l0, at which its contents are converted into a highlyturbulent fluidized mass M10 having an upper interface L10 and anapparent density of about 20-40lbs. per

cu. ft.

A dilute solids-in-gas suspension is withdrawn overhead from level L10and passed through a gas-solids separator, such as cyclone it, from,gas, at the conditions spec v d pcsitign about-as follows:

olsperce H2 A to 45 Total Cl (free and combined) 0.5

tb ir iulpur ti s T-Tg Any desired BQl'fiQQ-Qfi th ntellec gas may behea exchanged. with case s ccer 1 via Y lines 6 and '8 as abovedescribed. The total gas efiluent in line 22 may then be further cooledin heat exchange with feed. gases and/or other media in a heat exchangeand cooling system 24 to be passed through line 26 to a scrubber 30 at atemperature of about 60 to 200 F.

Returning now to generator l0, solid gasification residue is withdrawnthrough line 35 and passed to an extractor 40. Sea water may be suppliedthrough line 42 to extractor 40 in amounts adequate to extract the sodacontent of the ash. All or part of the sea water to be supplied to linemay be used for this purpose. Extracted ash may be discarded via line44. Enriched sea water is passed through line 46 to scrubber 30 whereinit is used to remove chlorine and its compounds from the product gas ina manner obvious to those skilled in the art. Sea water which may nowcontain NaCl somewhat above its normal concentration may be returned vialine 48 to feed line 5. The product gas recovered from scrubber 30through line 50 is now substantially free of halogen and is ready as afeed gas to a conventional hydrocarbon synthesis or other hydrogenationunit, if desired after further desulfurization by conventional means.However, scrubber 38 may also be so operated that a substantialproportion of the sulfur content of the product gas is absorbed therein.

The above description and exemplary operations have served to illustratespecific embodiments of the invention but are not intended to belimiting in scope.

What is claimed is:

1. In the production of ga mixtures containmetal halide yielding areaction product selected from the group consisting of elemental halogenand volatile halogen compounds at the reaction conditions said halidebeing present in amounts providing a halogen equivalent corresponding tothat of about 0.01-1% by weight of alkali metal halide based on carbonpresent and said amounts being adequate to inhibit the formation of CO2in favor of CO formation.

2. The process of claim 1 in which said halide is an alkali metalhalide.

3. The process of claim 2 in which said halide is sodium chloride.

4. The process of claim 1 in which said halide is added to saidmaterials prior to said reaction.

5. The process of claim a in which said halide is added in the form of anatural halide brine.

6. The process of claim 5 in which said'brine comprises sea water. 1

7. The process of claim 1 in which solid gasification residue withdrawnfrom said zone is ex- "6 treated with anaq eousm l m issolving compoundsof the cation of said halide-to iorma solution enrichedin said compound.

.8. The method of. claim 7 which said gas mixture isscrubbedwith saidsolution to remove at lea-eta substantial portion of itscontent inhalogen and volatile .halogen compounds from said gas mixture and toform .saidhalide in said. solution.

;9. The process of claim 8, in which the halide termed in said solutionisv introduced into said zone.

10. The process of claim lin which said halide is an alkaline earthmetal halide.

11. The process of claim 10- in which said halide is selected from thegroup consisting of the halidesof barium, calcium and magnesium.

12. In the production of gas mixture containing Hz and CO by reactingsolid carbonaceous materials with steam and oxygen in a gasificationzone at gasification conditions, the improvement which comprisescontacting said steam and oxygen simultaneously at said conditions insaid zone adding to said zone an extraneous metal halide forming at saidconditions a. reaction product selected from the group consisting ofelemental halogen and volatile halogen compounds, with a dense turbulentmass of subdivided carbonaceous solids fluidized by upwardly flowinggases to resemble a boiling liquid, said halide being present in amountsproviding a halogen equivalent corresponding to that of about 0.0l1% byweight of alkali metal halide based on carbon present and said amountsbeing adequate to inhibit the formation of CO2 in favor of CO formation.

13. In the production 01 gas mixtures containing H2 and CO by reactingsolid carbonaceous materials simultaneously with steam and oxygen in agasification zone at gasiflcation conditions, the improvement whichcomprises contacting said steam and oxygen at said conditions in saidzone with a dense turbulent mass of subdivided carbonaceous solidsfluidized by upflowing gases to resemble a boiling liquid and adding tosaid mass a natural metal halide brine containing a halide forming atsaid conditions a reaction product selected from the group consisting ofelemental halogen and volatile halogen compounds, said metal halidebeing present in amounts providing a halogen equivalent corresponding tothat of about 0.01-1% by weight of alkali metal halide based on carbonpresent and said amounts being adequate to inhibit the formation of CO2in favor of CO formation.

14. The process of claim 13 in which said brine comprises sea water.

15. In the production of gas mixtures containing H2 and CO by reactingsolid carbonaceous materials simultaneously with steam and oxygen in agasification zone at gasification conditions, the improvement whichcomprises impregnating subdivided carbonaceous solids of fluidizableparticle size with sea water of enriched metal halide content, saidmetal halide forming at said conditions a reaction product selected fromthe group consisting of elemental halogen and volatile halogencompounds, said metal halide being present in amounts providing ahalogen equivalent corresponding to that of about 0.01-l% by weight ofalkali metal halide based on carbon present, said amounts being adequateto inhibit the formation of CO2 in favor of CO formation contacting saidimpregnated solids with steam and oxygen at said con- 7 ditions in saidzone in the form of a dense turbulent mass of solids fluidized byupwardly flowing gases, withdrawing a product gas containing volatilehalogen compound and free halogen from said zone, separately withdrawingsolid gasification residue from said zone, leaching said residue withsea water to extract soluble alkali and alkaline earth metal compoundsand to produce sea water enriched in said last-named compounds,scrubbing said product gas with said lastnamed enriched sea water toextract volatile halogen compounds and free halogen from said productgas and to produce said sea water of enriched metal halide content, andusing said last- References Cited in' the file of this patent UNITEDSTATES PATENTS OTHER REFERENCES named sea water for said impregnatingtreat- 15 Grant: Chemical Dictionary. 3rd

ment.

WILLIAM J. MATTOX.

edition, page 685.

1. IN THE PRODUCTION OF GAS MIXTURES CONTAINING H2 AND CO BY REACTINGSOLID CARBONACEOUS MATERIALS SIMULTANEOUSLY WITH STEAM AND OXYGEN IN AGASIFICATION ZONE, THE IMPROVEMENT WHICH COMPRISES ADDING TO SAIDREACTION AN EXTRANEOUS METAL HALIDE YIELDING A REACTION PRODUCT SELECTEDFROM THE GROUP CONSISTING OF ELEMENTARY HALOGEN AND VOLATILE HALOGENCOMPOUNDS AT THE REACTION CONDITIONS SAID HALIDE BEING PRESENT INAMOUNTS PROVIDING A HALOGEN EQUIVALENT CORRESPONDING TO THAT OF ABOUT0.01-1% BY WEIGHT OF ALKALI METAL HALIDE BASED ON CARBON PRESENT ANDSAID AMOUNTS BEING ADEQUATE TO INHIBIT THE FORMATION OF CO2 IN FAVOR OFCO FORMATION.